White dwarfs undergoing hydrogen shell burning in single degenerate binary systems

We review recent X-ray observations of potential type Ia supernova progenitors: white dwarfs in "single degenerate systems" that are undergoing hydrogen shell burning. Grating spectra prove to be very different from each other, and basically fall into two classes: in some systems we observe X-ray emission in a wind, or diffuse circumstellar material, while in others we detect the white dwarf atmosphere. When the white dwarf atmosphere can be observed atmospheric models show promising preliminary results. The light curves offer also important information. Short term light curves of two post-outburst novae have revealed non-radial oscillations of the white dwarf. Long term light curves, possibly coordinated with optical observations, are very important to determine when the transient behaviour of supersoft X-ray sources is due to an interplay of mass accretion rate variations, atmospheric expansion and irradiation of the disk, and when we are instead observing hydrogen shell flashes. Another important open question concerns the X-ray light curve of post-outburst recurrent novae. Recent observations of IM Nor seem to rule out RN as statistically significant SNe Ia progenitors.